Asynchronous Transfer Mode (ATM) is the world's most widely deployed backbone technology. This standards-based transport medium is widely used within the core, at the access points, and in the edge of telecommunications systems to send data, video and voice at ultra high speeds.
ATM is a connection-oriented transfer mode that transmits user data in units called cells. Before sending cells a virtual connection between source and destination has to be established. The virtual connection is described in terms of Virtual Paths (VP) and Virtual Channels (VC). Basically a single VP has multiple VCs. All the cells of a connection follow the same path within the network. During the connection setup each switch on the path generates an entry in the Virtual Path Identifier (VPI)/Virtual Channel Identifier (VCI) translation table. This enables the switch to move an incoming packet from its VP/VC to corresponding outgoing VP/VC.
ATM Networks are able to transmit data with varying characteristics depending on the Quality of Service (QoS) desired. Different applications need different QoS. Some applications like telephony may be very sensitive to delay, but rather insensitive to loss, whereas others like compressed video are quite sensitive to loss. ATM specifies several Quality of Service (QoS) categories: CBR (Constant Bit Rate); rt-VBR (real-time Variable Bit Rate); nrt-VBR (non-real-time Variable Bit Rate); ABR (Available Bit Rate); and UBR (Unspecified Bit Rate).
A CBR connection reserves a constant amount of bandwidth during a connection setup. The source is allowed to send at the negotiated rate any time and for any duration. It may temporarily send at a lower rate as well. Applications that require small delay variations (jitter), such as voice, video and circuit emulation benefit from a CBR connection.
VBR negotiates a Peak Cell Rate (PCR), a Cell Delay Variation Tolerance (CDVT), a Sustainable Cell Rate (SCR) and a Maximum Burst Size (MBS) or Intrinsic Burst Tolerance (IBT). Sources that do well with VBR tend to be bursty such as compressed voice and video. Such applications can withstand larger delay variations (jitter). VBR service is further divided in real-time VBR (rt-VBR) and non-real-time VBR (nrt-VBR). They are distinguished by the need for an upper bound delay (Max CTD). MaxCTD is provided by rt-VBR, whereas for nrt-VBR no delay bounds are applicable.
ABR and UBR services use the remaining bandwidth, which is dynamic based on the bandwidth used by the CBT and VBR services. ABR and UBR transfer data that do not require tight constraints on end-to-end delay and delay variation. Typical applications are computer communications, such as file transfers and e-mail. UBR service provides no feedback mechanism, meaning that if the network is congested, UBR cells may be lost. An ABR source gets feedback from the network. The network provides information about the available bandwidth and the state of congestion. The source's transmission rate is adjusted based on the feedback information. This more efficient use of bandwidth alleviates congestion and cell loss. For ABR service, a guaranteed minimum bandwidth (MCR) is negotiated during the connection setup negotiations.
When a request for connection to the network is submitted, the request includes a description of the proposed traffic (traffic parameters) and the desired service (QoS parameters). The network performs admission control to determine if sufficient resources are available to satisfy the proposed QoS without violating the QoS of existing connections. Assuming that both conditions can be met, the network accepts the request.
Traffic parameters describe inherent characteristics of a traffic source. Traffic parameters include: Peak Cell Rate (PCR) which is an upper bound for cell rate; Sustained Cell Rate (SCR): which is the average cell rate measured over some long interval; Cell Delay Variation Tolerance (CDVT) which describes the maximum burst size that can be sent at the MaxCR; Maximum Burst Size (MBS) which is the maximum burst size that can be sent at the peak rate; and Minimum Cell Rate (MCR) which is the minimum cell rate a user is allowed to send.
ATM QoS parameters characterize performance of an ATM layer connection. QoS parameters include: Max Cell Transfer Delay (CTD) which is the delay experienced by a cell between network entrance and exit points; peak-to-peak Cell Delay Variation (CDV) which is the max-min cell transfer delay; Maximum Cell Rate (MaxCR) which is the maximum capacity usable by connections belonging to the specified service category; and Cell Loss Ratio (CLR) which is the percentage of cells that can be lost.
The network performs policing and shaping at the network entrance to ensure that the user's data flow adheres to the agreed upon specification. ATM's conformance definition defines conformity at an interface with respect to traffic contracts according to one or more instances of the Generic Cell Rate Algorithm (GCRA). The GCRA is often referred to as the “leaky bucket.” The GCRA allows the passing of cells so long as there is space in the “bucket” to accept the cells. The “bucket” leaks cells onto the network at a specified rate, thereby creating more space for accepting additional cells. Any cells that arrive at the bucket when it is full are discarded.
Mathematically, the GCRA is defined using Equation 1:
                                                        GCRA              ⁡                              (                                  T                  ,                  τ                                )                                                                                        t              :                              cell                ⁢                                                                  ⁢                arrival                ⁢                                                                  ⁢                time                                                                                        tat              :                              theoretical                ⁢                                                                  ⁢                arrival                ⁢                                                                  ⁢                time                                                                                        if              ⁢                                                          ⁢                              (                                  t                  <                                      tat                    -                    τ                                                  )                                                                                                                                    ⁢                              cell                ⁢                                                                  ⁢                nonconforming                                                                          else                                                                                                          ⁢                              cell                ⁢                                                                  ⁢                conforming                                                                                                                                    ⁢                              tat                =                                                      max                    ⁡                                          (                                              t                        ,                        tat                                            )                                                        +                  T                                                                                        EQUATION        ⁢                                  ⁢        1            
GCRA(T, τ) works as follows: For every new cell the arrival time is compared to the Theoretical Arrival Time (TAT). If the cell is late (i.e. the cell rate is lower than its nominal rate), it is accepted by the ATM network. If the cell is early a check is made to make sure that the deviation is within the tolerable limit, if so, the cell is accepted by the ATM network otherwise the cell is discarded.
The GCRA is used to define, in an operational manner, the relationship between Peak Cell Rate (PCR) and Cell Delay Variation Tolerance (CDVT) and the relationship between Sustained Cell Rate (SCR) and Burst Tolerance (BT). The CDVT is defined in relation to the Peak Cell Rate but is not a traffic descriptor (i.e. cannot be specified by the user). There is actually an assumption that PCR sources strictly conform to the PCR rate (i.e. GCRA(T, τ)) and that CDVs are added by local and/or public ATM switches.
Testing network traffic against a GCRA may reveal whether the traffic exceeds a contract and the number of cells that are in excess for either part of the contract (PCR or SCR). It is important to note that simply testing traffic against a GCRA qualifies whether the traffic complies, but does not “quantify” the traffic or indicate how close it is to complying (or not complying). If use of the GCRA is the only testing performed, the ideal contract parameters must of necessity be established by trial and error.
A variety of network test systems, such as the AGILENT TECHNOLOGIES J6800 series of Network Analyzers and the E4334A Broadband Series Test System, have evolved to provide full and uniform testing of ATM networks with high level performance measurement and protocol analysis. One of the key measurements taken by known network testing systems is the Cell Delay Variation measurement as defined by the GCRA. Such a measurement provides an indication of the level of jitter from a specified cell arrival time. However, the Cell Delay Variation measurement is only useful for CBR applications. By definition, the CDV measurement is in reference to the negotiated peak cell rate without regard to burst tolerance or sustained cell rate. VBR applications rely on a relatively high PCR and a lower SCR while CBR applications need only a single rate, the PCR (since their bit rate is supposed to be constant). Thus, known CDV measurements center around a specific cell rate. The variance measurement is based on the time delta around that specific cell rate. In a VBR connection there can be several cell rates that are of interest.
The present inventors have recognized a need for apparatus and methods for quantifying and presenting cell arrival times that allow a user to quickly identify and manage contract parameters for constant-bit-rate-traffic and variable-bit-rate traffic.